Oil and gas well drilling operations commonly include the use of sensors deployed downhole as a part of the drill string to acquire data as the well bore is being drilled. This real-time data may provide information about the progress of the drilling operation and the earth formations surrounding the well bore. Significant benefit may be obtained by improved control of downhole sensors from the rig floor or remote locations. For example, the ability to send commands to downhole sensors that selectively activate the sensors can conserve the battery life of the sensors and increase the amount of downhole time a sensor is useful.
Directional drilling operations are particularly enhanced by improved control. The ability to efficiently and reliably transmit commands from a driller to downhole drilling hardware can be essential, in many situations, to accurate well bore positioning. Downhole drilling hardware that, for example, deflects a portion of the drill string to steer the drilling tool is typically more effective when under tight control by a driller through the ability to continuously adjust the projected direction of the well path by sending commands to the downhole drilling hardware. This ability allows a driller to continuously interpret real-time data (e.g., survey data) received from downhole sensors and fine tune the projected well path accordingly. In such applications, reliable and accurate data transmission is important as errors in command interpretation by the downhole drilling hardware may cause considerable difficulties.
Some prior art communication mechanisms require that the drill string stop rotating and/or that the mud motors stop pumping prior to transmitting commands to the downhole tool. Such techniques tend to be disadvantageous since each time the drilling operation is stopped valuable rig time is lost. Moreover, stopping the drill string increases the likelihood that it becomes irretrievably lodged in the borehole. Prior art communication mechanisms that rely on absolute rotation rates of the drill string to encode data are known (including U.S. Pat. No. 5,603,386 to Webster). Such techniques are serviceable, but can be improved upon. For example, the optimum rotation rate of the drill string may vary within an operation, or from one operation to the next, depending on the type of drill bit being used and the strata being penetrated. Such techniques also typically require that the drill string be stopped prior to transmitting data.
Therefore, there exists a need for improved techniques for communicating from the surface to a downhole tool. In particular, there is a need for a technique that does not significantly interrupt the drilling operation and that is typically effective regardless of the preferred drilling rate.